There is no guarantee, but there is high probability that your transaction will get confirmed in a block. If the average tx with proper fee exceeds the block limit, the estimate moves higher. If you pay more than the minimum, or higher than the average or estimate, then you will get a higher probability of your transaction being confirmed sooner.

Right now:

estimatefee 25
0.00024126

estimatefee 2
0.00057960

So you just pay a tx fee higher than that lower number above and you'll be fine. If you pay a fee higher than the second number, your tx will simply be confirmed in an estimated 2 blocks time. This is from the bitcoin core estimatefee command.

A long time ago, the fee was 0.01, then lowered to 0.001. 0.0005 isn't that bad, it's still less than a dollar equivalent.